How to integrate enums in propertyWrapped UserDefault

Question

The struct below will work for primitive data types such as Int, String, Double, etc., Is there a way to make this work for enums so that I won't have to use rawValues an manual parsing?

@propertyWrapper
struct Storage<T> {
  let objectName: String
  let defaultValue: T
  let defaults: UserDefaults

  init(_ objectName: String, defaultValue: T, defaults: UserDefaults = .standard) {
    self.objectName = objectName
    self.defaultValue = defaultValue
    self.defaults = defaults
  }

  var wrappedValue: T {
    get { return self.defaults.object(forKey: self.objectName) as? T ?? self.defaultValue }
    set { self.defaults.set(newValue, forKey: self.objectName) }
  }
}

Currently my workaround is to wrap the object in another getter and setter for the enum like this.

enum SomeType: String {
  case foo, bar, baz
}


class Defaults {
   @Storage("object", default: "")
   private var: objectContainer: String

   var object: SomeType? {
      get {
         return SomeType(rawValue: self.objectContainer)
      }
      set {
         self.objectContainer = newValue.rawValue ?? ""
      }
   }
}

I tried to create a different property wrapper specific for RawRepresentable types but Swift can't infer the type of T from that.

@propertyWrapper
struct RawRepresentableStorage<T: RawRepresentable> {
  let objectName: String
  let defaultValue: T
  let defaults: UserDefaults
  
  init(_ objectName: String, defaultValue: T, defaults: UserDefaults = .standard) {
    self.objectName = objectName
    self.defaultValue = defaultValue
    self.defaults = defaults
  }
  
  var wrappedValue: T {
    get {
      guard let object = self.defaults.object(forKey: self.objectName) as? T else {
        return self.defaultValue
      }
      
      // Error on this line:
      // `Cannot convert value of type 'T' to expected argument type 'T.RawValue'`
      return T(rawValue: object) ?? self.defaultValue 
    }
    set {
      self.defaults.set(newValue.rawValue, forKey: self.objectName)
    }
  }
}

It also wouldn't be ideal to create Wrapper types for each storable enum within the codebase even if there are just a handful of them

Is there something I'm missing or is this a current limitation of Swift?

Answer 1

In your wrappedValue getter, you are casting to the wrong type. You are passing object to T(rawValue:), so its type needs to be T.RawValue, not T.

@propertyWrapper
struct RawRepresentableStorage<T: RawRepresentable> {
    let objectName: String
    let defaultValue: T
    let defaults: UserDefaults
    
    init(_ objectName: String, defaultValue: T, defaults: UserDefaults = .standard) {
        self.objectName = objectName
        self.defaultValue = defaultValue
        self.defaults = defaults
    }
    
    var wrappedValue: T {
        get {
            guard let object = self.defaults.object(forKey: self.objectName) as? T.RawValue else {
                return self.defaultValue
            }
            
            return T(rawValue: object) ?? self.defaultValue
        }
        set {
            self.defaults.set(newValue.rawValue, forKey: self.objectName)
        }
    }
}